3D model (JSmol)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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The tetrapyrrole called hydroxymethylbilane is normally converted by the action of uroporphyrinogen-III synthase to uroporphyrinogen III. Uroporphyrinogen III is subsequently converted into coproporphyrinogen III and on to heme. If, however, no uroporphyrinogen-III synthase is present, hydroxymethylbilane will spontaneously cyclise into uroporphyrinogen I, which is then converted into coproporphyrinogen I, also by uroporphyrinogen III decarboxylase.
The difference between the two forms, is the arrangements of the four carboxyethyl ("P" groups) and the four carboxymethyl groups ("A" groups). The non-enzyamtic conversion to uroporphyrinogen I results in the sequence AP-AP-AP-AP, whereas the enzymatic conversion into uroporphyrinogen III lead to reversal of one AP-group and hence an AP-AP-AP-PA arrangement.
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